1.245 Pythagorean comma

Originally, the interval by which the sum of six whole tones exceeds the octave
– (9:8)^6 - 2:1 = 531441:524288, or 23.5 cents.

Modern acoustical theory defines it as the interval by which twelve fifths
exceed seven octaves. To put it another way: A sequence of fifths that starts
on C eventually circles back to C. However, this C is 23.5 cents higher
than the C obtained by adding 7 octaves. The difference between those two
pitches is the Pythagorean comma.